RANGE EXPANSION OF THE CARDINAL AND OTHER BIRDS IN THE NORTHEASTERN STATES

BARBARA G. BEDDALL

T HE populations of the Cardinal (Richmondena cardinalis) and the Tufted

Titmouse (Parus bicolor) have recently “exploded” in southern New

England and the adjacent Hudson River Valley. The Carolina Wren (Thryo-

thorus ludovicianzss) and the Mockingbird (Mimns polyglottos) are also

undergoing a range expansion in the same area, although to a much slighter

degree. Both the Cardinal and the Tufted Titmouse are newcomers as breed-

ing birds in the region east of the Hudson River and north of Long Island

Sound. The Mockingbird, on the other hand, seems to be reinvading a terri-

tory it occupied several hundred years ago (Merriam, 1877). The Carolina

Wren became established as a breeding bird in Stamford, Conn. about 1895

(Sage, 1913).

It is the purpose of this paper to investigate the characteristics of these

range and population changes and to examine possible reasons for them.

SOURCESOFDATAAND METHODS OF STUDY

Audubon Christmas Counts have been used as a basis for determining popu-

lation and range changes. Stewart (1954) has considered carefully many

valid objections to these figures. He concludes that their “application . . . should be restricted to the indication of trends in population. . . .” With this

restriction in mind, one notes that the figures do present a plausible and

consistent picture, one not analyzable by refined mathematical techniques, but

one which is nevertheless useful as a point of departure. Moreover, the

general population trends so shown are substantiated by independent observa-

tions noted in Audubon Field Notes, Records of New England Birds, and other

local ornithological publications.

Local or regional Christmas Count figures have been converted to numbers

of birds seen per 100 party hours. (Stewart would prefer a conversion based

on mileage rather than time, unfortunately an impossibility under present

reporting methods.) No other adjustments have been attempted because of

the difficulties in applying them equably.

Table 1 shows numbers of birds seen per 100 party hours for the four

species for several contrasting regions. “Southern New England” includes

the states of Connecticut, Massachusetts, and Rhode Island, and Long Island

and the Hudson River Valley in New York State as well.

Figure 1 illustrates the changes in distribution and abundance of the four

species from I945 to 1960 in the area defined as “Southern New England.”

Figures 2 and 3 show details of individual counts for the Cardinal and the

140

1.42 THE WILSON BULLETIN June 1963 Vol. 75, No. 2

FIG. 1. Changes in distribution and abundance of four species of birds, 1945-1960, in Connecticut, Massachusetts, Rhode Island, and in Long Island and the Hudson River Valley, New York, based on Audubon Christmas Counts. Rare: l-10 birds per 100 PH (party hours) ; Uncommon: lo-50/100 PH; Common: SO-250/100 PH ; Abundant: over

250/100 PH.

Barbara G. Bddall

RANGE EXPANSION OF CARDINAL 143

- 300

= 300

- 200

= 100 = = - - 30 - 20

NEW CONN.

FIG. 2. Rate of population growth of the Cardinal, 1945-1960, in southeastern New

York and southwestern Connecticut, based on Audubon Christmas Counts (see text).

Last bars on right refer to 1960.

144 THE WILSON BULLETIN Jnne 1963 Vol. 75, No. 2

. D

J l Pu

- 500 -I; = 300 - 200

= loo = =

= 30 R’

J :I NEW YORK 1

.

1 :

Pe

- 20

s 10

=5

If

-2

-I

CONN.

I . Wd

FIG. 3. Rate of population growth of the Tufted Titmouse, 194551960, in southeastern

New York and southwestern Connecticut, based on Audubon Christmas Counts (see text).

Last bars on right, or eingle bars, refer to 1960.

Barba,a G. Be&la11

RANGE EXPANSION OF CARDINAL 1‘45

Tufted Titmouse in the area that includes the lower Hudson River Valley

(Bronx-Westchester Region, Dutchess Co., Kingston-Lake Mohonk, Peekskill,

Putnam Co., Rockland Co.), northern Long Island (Northern Nassau Co.,

Smithtown), and southwestern Connecticut (Greenwich-Stamford-Port-

Chester, Litchfield Hills, Oxford, Waterbury, Westport). A logarithmic scale

has been used to indicate rate of growth.

RANGE AND POPULATION CHANGES

The Cardinal population in Southern New England has increased spectacu-

larly in the past 15 years. Th is species first nested in Connecticut around

1943, and in eastern Massachusetts in 1958 (Audubon Field Notes) . Westward

across the Hudson River it is now a common year-round resident in Rockland

Co., N.Y., although 30 years ago it was confined to the southeastern corner of

this county (Rockland Audubon Society, 1959). The New Jersey population

has been increasing steadily for the past 20 years. In Pennsylvania there has

been an increase in the northeastern part of the state, although in general the

population has been relatively stable for the past 20 years (based on state

and local Christmas Counts). An earlier increase in this state is noted by

Sutton (1928). Relative stability has also been reached in Michigan, follow-

ing an increase that began about 1900 (Burns, 1958). The apparently related

increase in Ontario, dating from about 1938 (Snyder, 1951)) is now slowing

down. The population in South Dakota, on the other hand, shows a marked

decline in the last decade. The Cardinal population is also decreasing in

Pasadena, California, where the species was successfully introduced in 1923

(Grinnell and Miller, 1944). At its present rate of decline it appears headed

for extinction there.

The Tufted Titmouse is much less widely distributed in Southern New

England than the Cardinal, but the pattern of its population growth is similar.

This species first nested in Connecticut in 1946 and in eastern Massachusetts

in 1958 (Audubon Field Notes). It has become established in Rockland Co.,

N.Y., within the past 30 years (Rockland Audubon Society, 1959). The

recent increases in New Jersey are actually confined to a few places in the

northern part of the state, while there has been little recent change in Penn-

sylvania.

The Carolina Wren population has also increased in Southern New England.

It is quite limited in both distribution and numbers, however, being largely

confined to areas along Long Island S ound. Earlier upward trends in New

Jersey and Pennsylvania have leveled off.

The Mockingbird population has increased slightly in Southern New Eng-

land but, at the rate of one bird seen per 100 party hours, it is certainly far

from common. On the other hand, the New Jersey and Pennsylvania popu-

146 THE WILSON BULLETIN Junr 1963 Vol. 75, No. 2

lations have increased as sharply as have those of the Cardinal and Tufted

Titmouse in New England, although the numbers are smaller. An earlier in-

crease in Maryland dates from about 1900 (Stewart and Robbins, 1958). A population buildup within the original range preceding an extension of

the range was suggested by Odum and Johnston (1951) as a factor in the

southward movement of the House Wren. Population pressure does appear

to play a part in the northward movement of the four species considered here

(see Table 1; Figs. 1 and 2). Such a buildup and the resulting population

pressure may also lead to the successful crossing of geographical barriers and

the invasion of previously isolated territory (see below). The conditions

encountered in such new and isolated territory must be unusually favorable to

permit more than temporary occupation of it. This seems to have been true

for the Cardinal and the Tufted Titmouse in Southern New England. The

rapid increase in their numbers is still continuing, resulting in fairly large,

native populations which are no longer dependent on irregular immigration. Furthermore, such a rapid increase implies a high survival rate, greater

success m rearing young, an increase in clutch size or number of broods, or

some combination of these factors. It is possible that the Cardinal may be

more successful in rearing a large brood in this area where the population

density is still comparatively low and intraspecific competition presumably

less intense. Four live young have been reported several times in New England

(Audubon Field Notes). At a similar latitude in southeastern Michigan where

the Cardinal is common, Sutton (1959) f ound that only three eggs hatched in

the two nests where four were laid. Three successful broods have been re-

ported for a pair of Cardinals and two for a pair of titmice in Massachusetts

(Audubon Field Notes), both high for the species involved. Another point is the rapidity of the response to changing environmental

conditions. Since time would be required for the population buildup, Odum

and Johnston (1951) concluded that a time lag would exist between changing

biotic conditions and range extension. This lag may also exist in connection

with climatic changes (see below).

FACTORS INFLUENCING RANGE AND POPULATION CHANGES

Relevant characteristics of the birds are their general adaptation and

adaptability and their general mobility ; significant environmental factors are

climate and weather, available habitat, geographical barriers, and man. There

is constant interaction among all these factors whose relative importance also

varies according to species, location, etc.

Adaptation and adaptability.-Here is meant the general type of habitat for

which the species is adapted and its ability to make use of changed conditions.

The Cardinal, Carolina Wren, and Mockingbird show a preference for thick-

RANGE EXPANSION OF CARDINAL 147

ets, tangles, underbrush, and shrubbery (Bent, 1948; Burleigh, 1958; Hund-

ley, 1953; Laskey, 1944; Pough, 1946). Th us, activities of man which increase

this sort of habitat should be beneficial to these species. This seems to be

true for the Cardinal and Mockingbird, which also show great adaptability

to the presence of man himself. Wilson (1831) noted that all except the

Carolina Wren were common around farm houses, particularly in winter.

The behavior of the Carolina Wren, the least common of the four species,

shows considerable ambivalence (Bent, 1948; Pough, 1946). Some indi-

viduals are willing to accommodate themselves to man as a close neighbor and

others are not. Certainly this species’ chances of long-term survival would be

improved if its acceptance of man were increasing.

The Tufted Titmouse is largely a forest bird (Pitelka, 1941)) although it

may wander in more open areas outside the nesting season. It shows great

adaptability to man himself, being a frequent visitor at feeding stations. But

its general adaptation to a forest habitat means that its success is limited by

the availability of this habitat.

Mobility.-Powers of dispersal are naturally important in a discussion of

range extension. All four species are considered to be nonmigratory, but

a certain amount of movement does exist and can be demonstrated in various

ways. Banding records for individual birds indicate a minimum amount of

movement for the Cardinal (Geis, pers. corn.; Hundley, 1953; Laskey, 1944;

Lincoln, 1939; Stewart and Robbins, 1953). In general, those recovered have

been found within a distance of five miles of the banding station, although

long distance records do exist. The Titmouse is found within an even smaller

area (Van Tyne, 194s). The M oc ingbird has been recovered at greater k

distances (Lincoln, 1939). This indeed seems a necessary corollary to the

small, widely scattered population in New England.

Of equal interest are the birds which are never seen again, generally a

rather high percentage of those banded. Either they have died, become trap-

shy, or moved on. That there must be a good deal of movement, particularly

during the winter, is shown by the marked seasonality of the appearance and

disappearance of Cardinals where banding has taken place at one location

over a period of time (Geis, pers. corn. ; Hundley, 1953; Laskey, 1944). The

largest numbers of new birds are banded in the months from November to

February. This point is also corroborated by Records of New England Birds,

which reports twice as many Cardinals seen in these same months as in the

remainder of the year.

Another method of demonstrating movement is to compare occurrence and

population density in the winter-bird and breeding-bird censuses published in

Audubon Field Notes (see Table 2). No attempt has been made to compare

habitats because of the many difficulties involved in classifying them. How-

148 THE WILSON BULLETIN June 1963 Vol. 75, No. 2

TABLE 2

COMPARISON OF WINTER-BIRD AND BREEDING-BIRD CENSUSES, 1956-1960*

Species

Tufted Titmouse Carolina Wren Mockingbird Cardinal

Winter-bird censures Breeding-bird censuses* *

Present on Over 13/100 acres Present on Over 13/100 acres

NO. %t No. %$ No. %t NO. lo:

75 51.3 20 26.7 75 40.0 38 52.0 51 35.0 5 9.8 60 32.8 17 28.4 25 17.3 2 8.0 16 8.75 3 18.8 98 67.0 11 11.2 88 47.0 51 59.2

* From Audubon Field Notes. ** Pairs converted to individuals, young not included.

t Per cent of total censuses. f Per cent of cen~me~ where present.

ever, it is clear that for each species the winter populations are both more

widely dispersed (with the possible exception of the Carolina Wren) and less

dense than the summer populations. The change in density is plainly shown by

the winter and summer censuses taken at two locations in El Dorado, Arkansas

(see Table 3). The winter decrease in density must be attributed more to dis-

persal than to mortality since the species are then found at more places, and the

succeeding increase in breeding-bird density does not include young of the

TABLE 3

COMPARISON OF WINTER-BIRD AND BREEDING-BIRD POPULATIONS, EL DORADO, ARKANSAS*

Number/100 acres

Tufted Titmouse Carolina Wren

Winter Breeding** Winter Breeding**

Year Upland pine and pine-oak woodland

1953 9 26 9 38 1954 9 30 9 30 1955 9 44 4 30 1956 13 48 13 26 1957 13 52 13 60

Mature oak-pine stream bottomland

1951 40 18 1952 9 54 9 40 1953 25 36 10 50 1954 15 112 20 50 1955 25 90 10 56 1956 25 60 25 122 1957 30 80 20 150

* From Audubon Field Notes. ** Pairs converted to individuals, young not included.

Cardinal

Winter Breeding**

+ 34 + 34 4 30 4 60 9 74

54 4 50 5 50

15 70 15 60 10 56 40 70

Barbara G. Bwldall

RANGE EXPANSION OF CARDINAL 149

I MASS.

I Western

I 46.0° I

I Central

\ 44.2O

FIG. 4. Climatic divisions, with mean annual temperatures, 1931-1955. From U.S. Weather Bureau data.

year. This implies a winter dispersal followed by a withdrawal (to more

favorable breeding areas?) before the breeding season begins.

Thus, a certain amount of exploratory activity occurs in all four species,

making range extension possible under advantageous conditions.

Climate and weather.-Little is known about the temperature tolerances of

birds. Nevertheless, it seems possible to draw certain general conclusions

from present distribution and abundance (based on local Christmas Counts).

These have been compared with the mean annual temperature of the climatic

divisions of the states as drawn by the U.S. Weather Bureau (1959, 1960).

Figure 4 summarizes this information for Southern New England.

The Cardinal is the most temperature tolerant of the four species. It is com-

mon in all the climatic divisions of Pennsylvania with a mean annual tempera-

ture above 48 F, although it is also present in the two colder divisions, the

Pocono Mountains and the Upper Susquehanna. In Southern New England,

only northwest Connecticut (47 F) and western Massachusetts (4<6 F) fall

below 48 F, an indication that climatic conditions in most of this area are

generally suitable for the Cardinal. Here one might note that northwest

THE WILSON BULLETIN June 1963 Vol. 75, No. 2

Connecticut appears to be a barrier and that the Cardinal seems to be spread-

ing up the Hudson River Valley and the Connecticut River Valley more or less

independently (see Figs. 1 and 2).

The Tufted Titmouse is fairly numerous in all the climatic divisions of

Pennsylvania with a mean annual temperature of 50 F and above. On this

basis, one would expect the main population growth of the titmouse in New

England to be confined to coastal areas and the lower Hudson River Valley.

Northward movements into New Hampshire and even Maine were noted in

the fall of 1961 (Audubon Field Notes) ; it will be interesting to see if breed-

ing records follow.

The Carolina Wren is found in Pennsylvania and New Jersey where mean

annual temperatures range from 50 to 53 F, although it is more numerous

in the warmer sections. Again, one would expect the principal development

in New England to be in the coastal areas. It is commonly noted in the litera-

ture, however, that this species is subject to winter kill. Its numbers are very

erratic within a wide span along its northern boundary.

The Mockingbird is relatively common in those climatic divisions of Penn-

sylvania and New Jersey having mean annual temperatures of 53 F and above.

It seems unlikely, on this basis, that the Mockingbird will become common

in New England under present conditions.

All of these species appear to have definite climatic limitations. From this

it may be deduced that climatic warming would favor northward expansion

and, conversely, that climatic cooling would lead to a contraction of range

at the northern edge. Figure 5 shows the mean annual temperature above

and below the long-term average for the states of Connecticut, Massachusetts,

and Rhode Island combined, since 1900. The long-term trend has been up-

ward, although the most recent years subb uwest a reversal of this trend. The

trend for New Jersey follows a similar pattern. In Pennsylvania, however, the

peak was reached in 1931 and the trend has been downward ever since.

Drury (1957, 1958) notes the vegetational changes in old-field successions

in southern New England as a result of climatic warming. There has been a

northward expansion of red cedar (Juniperus virginiana) and gray birch

(Be&a popuZifoZia), and an accompanying northward retreat of new white

pine (Pinus Strobus) . In looking for relationships in the trends, one finds that the Cardinal be-

came established in Southern New England in the late 1940’s and the early

1950’s, during the long period of above average temperatures which began in

1931. The previous increases in New Jersey and Pennsylvania may well have

been sparked by the rising temperature trends there. It is noteworthy, how-

ever, that the Cardinal has continued to increase in New England while

weather conditions in recent years have become much more severe. From

RANGE EXP-4NSION OF CARDINAL 151

49.0"

48.50

48.00

36.0"

1900 1960

FIG. 5. Mean annual temperatures, 1900-1960, for Connecticut, Massachusetts, and Rhode Island, with deviations from long-term average (1888-1960). Based on U.S. Weather Bureau data.

this one may conclude that the mild period permitted the population to build

up to a size sufficient to withstand the recent harsher period. The less hardy

birds might not now survive, but this would not cause the destruction of a

sufficiently large and generally hardy population. The same reasoning may

be applied to the Tufted Titmouse and perhaps also to the Carolina Wren in

New England (though see above), and to the Mockingbird in New Jersey and

Pennsylvania.

In Michigan the temperature trend has been relatively steady for the past

30 years, following an earlier increase. The relative stability of the Cardinal

and Tufted Titmouse populations in this state may be a reflection of this

condition.

A long-continued temperature decrease might, however, cause a reversal

of these population trends. In South Dakota there was a strong upward tem-

perature trend, culminating in the year 1931, followed by a sharp decrease.

Mean annual temperatures averaged by decades show an increase from 44.6 F

for 1911-1920 to 47.1 F for 1931-1940, and a subsequent decrease to 45.6 F

for 1951-1960. In spite of this, both the breeding and wintering ranges of

the Cardinal continued to extend northward there into the early 1950’s

(Krause and Froiland, 1954). The population, however, has now begun to

decrease (see Table 1)) possibly as a delayed reaction to decreasing tem-

peratures.

152 THE WILSON BULLETIN June 1963 Vol. 75. No. 2

The decreasing temperature trend in Pennsylvania apparently has not

reached a point where it is causing any decrease in the well-established

Cardinal and Tufted Titmouse populations, although it may be affecting the

much more sparse population of the Carolina Wren and slowing down the

increase in the Mockingbird population.

If the Mockingbird really was an early inhabitant of New England, perhaps

one of the reasons for its extirpation was the long cold period extending from

lSl1 to 1904. This was interrupted by only three very short warmer periods

(based on the record at New Haven, Connecticut; Kirk, 1939). Connecticut

Mockingbird records going back to the 1860’s (Sage, 1913) fall generally

in the warmer periods. This long cold period may have affected the other

species adversely as well.

Hub&at.-Habitat changes have been extensive in the area under considera-

tion. Deforestation became important after 1800 as agriculture made increas-

ing demands on the land. This trend was reversed with the opening of the

West. The resulting farm abandonment has continued in New England to the

present day. One hundred years ago only 27 per cent of Connecticut land

was in forest. At the present time, 63 per cent of this land is covered by

forest, though much of it is young and scrubby. At the same time, there has

been a large increase in human population. This has caused other changes in

land use, ranging from rural to suburban to completely urban conditions.

With the exception of parks, the latter obviously are unsuitable for the four

species of birds, and in fact may act as a barrier to quite sedentary birds.

Where human population is less dense, the four species of birds have reacted

in different ways.

The Cardinal first became established in southwestern Connecticut in the

1940’s. Per cent of land used for agriculture in Fairfield County, Connecticut,

declined from 53 per cent in 1935 to about 8 per cent in 1959. The abandoned

farm land resulted in an increase in brush and young forest, i.e., habitat suit-

able for the Cardinal. At the same time, residential land use increased rapidly,

also providing habitat suitable for this species. Perhaps the “tip point” has

been reached here, however, for the most spectacular growth of Cardinal popu-

lations has taken place not in Fairfield County, Connecticut (human popula-

tion density l,O33/sq. mi.), but in Putnam County, New York (135/sq. mi.)

and northwestern Westchester County, New York. At the same time, it has

been noted that range expansion of the Cardinal has occurred in settled rather

than in unsettled areas in northern New Jersey (Fables, 1955) and in Michi-

gan (Burns, 1958). In other words, some human settlement is a favorable

factor, but a point may be reached where human density is too great.

The Carolina Wren and the Mockingbird are more limited climatically,

but there are further differences. The Carolina Wren has done comparatively

Barbara 6. Rdddl

RANGE EXPANSION OF CARDINAL 153

well in the less heavily settled areas on Lon g Island Sound, confirming previ-

ous remarks on its rather poor adaptability to the presence of man. The

scarcity of the Mockingbird must be due more to climate, since it is the most

“domesticated” of the four.

The Tufted Titmouse is quite limited by its habitat requirements. Although

much of the forest in the oak-hickory region of Southern New England is

young (Thomson, 195S), enough older forest apparently exists to allow the

titmouse a foothold. The increase in the Tufted Titmouse population in New

Jersey is largely confined to areas of less dense human population in the

northern part of the state. Increasing warmth has not led to an increase in

titmice where the habitat is not suitable.

Food supply does not appear to be an important limiting factor at present

for any of the species, with the probable exception of the Carolina Wren.

This species is almost completely insectivorous throughout the year. The

Cardinal, Tufted Titmouse, and Mockingbird are typical of the songbirds

wintering in New England in eating a large proportion of seeds, nuts, and

fruits at this time of year (Martin, Zim, and Nelson, 1951). However, the

reportedly poor acorn crop in 1961 may have been a factor in the northward

expansion of the Tufted Titmouse in the fall of that year. At times of short

supply, the prevalence of feedin g stations may be valuable to the Cardinal

and Tufted Titmouse, both of which are constant winter visitors.

Geographical barriers.-Geographical features such as mountains, rivers or

other bodies of water, and metropolitan areas (man-made, but a fact of

geography all the same) may have opposite effects on range extension, de-

pending particularly on population pressure.

The importance of these features as barriers can be deduced from various

sources. None of these species is common in the high northwestern section

of New Jersey (local Christmas Counts; Fables, 1955) . In addition, many

Christmas Counts have been made at various altitudes in the Appalachian

Mountains. Arranged according to decreasing importance of altitude as a

barrier, the four species may be listed as follows: Mockingbird, Carolina

Wren, Cardinal, Tufted Titmouse. None of them reaches the highest points.

The importance of water as a barrier can also be deduced from the Christmas

Counts (see Figs. l-3). Again arranged according to decreasing importance

of the barrier, the species may be listed as follows: Tufted Titmouse, Cardinal,

Carolina Wren, and Mockingbird. As recently as 1942, Cruickshank con-

sidered the Tufted Titmouse to be only a rare possibility east of the Hudson

River or on Long Island. The population buildup in Rockland County, New

York began about this time and led to the successful crossing of the Hudson

River. The subsequent increase in southeastern New York and southwestern

Connecticut has apparently resulted in its re-invasion of Long Island, where

154 THE WILSON BULLETIN June 1963 Vol. 75, No. 2

it is once again listed as a breeding bird (Buckley, 1961). It seems probable

that the Cardinal followed the same route, although it is impossible to prove

this without banding data. Rivers may be barriers, while their valleys may

be pathways for expansion. Th e c imate 1 of the Hudson River Valley is milder

than that of the higher lands on either side and thus could be expected to be

more suitable for climatically limited species.

The land passageway north from New Jersey is further narrowed for the

Cardinal and the Tufted Titmouse by the New York metropolitan region.

A small population in New Jersey that has limited access to new territory

may find northward expansion impossible. But population pressure may turn

these barriers into a funnel leading to the successful invasion of new territory.

This seems to apply to the Cardinal and the Tufted Titmouse, for both of

which New England was a relatively isolated area. On the other hand, New

England has long been within the reach of both the Carolina Wren and the

Mockingbird. Their more modest success seems related rather to less suitable

climatic and/or habitat conditions.

IMan.-All four species were present on Long Island 100 years ago (Giraud,

1844). By about 1900, the Cardinal had become uncommon around New

York City, the Mockingbird was rare? and the Tufted Titmouse only accidental

on Long Island (Chapman, 1906; Cruickshank, 1942; Eaton, 1910; Griscom,

1923). The Mockingbird had disapp eared from the Philadelphia area as

early as 1830 (Wilson, lS31), and near the turn of the century was listed as

very rare in eastern Pennsylvania and New Jersey (Stone, 1894). Both the

Cardinal and the Mockingbird were called shy and difficult to approach by

Gentry (1876).

Much of this reduction in numbers seems due to direct intervention by man.

Both the Cardinal and the Mockingbird were popular as cage birds, at prices

ranging from ten dollars and up per bird. Songbirds were used as food and

for decorative purposes. Hunting and trapping would thus have been im-

portant in limiting numbers, especially at the edges of ranges. Early deforesta-

tion posed an equally serious threat to the Tufted Titmouse. These much-

reduced populations would have made settlement of new areas unlikely, even

if they were suitable. The relaxation of hunting pressure has undoubtedly

contributed to the present increase in Cardinal and Mockingbird populations,

while the renewed growth of the forest has aided the Tufted Titmouse.

SUMMARY AND CONCLUSION

Granting the imperfections in the data, as previously noted, it still seems

possible to venture some conclusions.

The Cardinal has become a common-to-abundant resident in southern New

England and the lower Hudson River Valley within the past 15 years. Al-

RANGE EXPANSION OF CARDINAL

though resident in the area around New York City 100 years ago, it had he-

come very rare there by 1900, probably as a result of hunting pressure and

of the long cold period in the 1800’s. Increasin g temperatures and a relaxation

of hunting pressure, plus sufficient suitable habitat, led to a population in-

crease in New Jersey. The resulting population pressure brought about the

successful crossing of the Hudson River. The firm establishment of the popu-

lation in southern New England occurred during a period of maximum

warmth. With suitable habitat available, it seems that the Cardinal will con-

tinue to prosper in southern New England and the lower Hudson River Valley,

threatened only by a long-continued temperature decrease or a drastic change

in habitat.

The Tufted Titmouse has followed a similar pattern, although it is more

limited by climatic and habitat requirements than is the Cardinal and is,

therefore, not likely to achieve as great a success in southern New England.

Change in amount of suitable habitat (deforestation and reforestation) was

a more important influence than hunting pressure. The titmouse seems firmly

established in southern New England and, with the increasing age of forests,

may even have added habitat available. Its future here seems fairly secure,

barring the leveling of the forests or a long-continued temperature decrease.

The Carolina Wren is apparently responding to changes in temperature and

is succeeding moderately well along Long Island Sound. At the same time, it

seems to be the most sensitive of the four species to the presence of man. Its

stricter temperature and habitat requirements do not pressage a bright future

for it in New England. Furthermore, increasing human population density,

which seems a fairly certain prospect, would militate against it; decreasing

temperatures would have an additional depressing effect.

The Mockingbird’s record is similar to the Cardinal’s, although on a more

elongated scale geographically. Its northeastern population was reduced both

by hunting and by decreasing temperatures, and the reversal of these condi-

tions has led to an increase in numbers. It is the most limited climatically of

the four species but, at the some time, apparently the most wide-ranging.

Therefore, a small, though not necessarily self-sustaining, population in New

England is a likelihood, while any great increase is an improbability. A long-

continued temperature decrease would be detrimental.

Range extension of these sedentary species seems to be preceded by a popu-

lation increase within the original range. This increase may be stimulated

by climatic warming (all four species), habitat changes (favorable for all

but the Carolina Wren), or relaxation of hunting pressure (Mockingbird,

Cardinal). A time lag in responding to these changes, particularly the climatic

changes, is evident. Possibilities for range expansion, given proper conditions,

1.56 THE WILSON BULLETIN June 1963 Vol. 75. No. 2

are always present in the winter exploratory activity exhibited by all four

species.

The pressure resulting from these population increases may lead to gradual

range extension where geographical barriers are unimportant and the new

area is more or less suitable (Carolina Wren, Mockingbird). The results may

be more dramatic, however, when this pressure leads to the invasion of a

suitable area from which the species had been excluded by geographical

barriers (Tufted Titmouse, Cardinal).

It does not seem necessary to postulate evolutionary changes in these species

of birds to account for the range and population changes discussed above.

The species are occupying areas in southern New England and the Hudson

River Valley that might be expected on the basis of their distribution else-

where and that seem in accord with biotic and climatic changes. However,

the new, rapidly expanding and relatively isolated populations of both the

Cardinal and the Tufted Titmouse might well result in evolutionary changes.

This possibility requires further study.

ACKNOWLEDGMENTS

Among the many people who helped with this paper and whom I particularly want

to thank are Professor G. Evelyn Hutchinson and Dr. Philip S. Humphrey. For reading

and criticizing the manuscript I want to thank Dr. Edward S. Deevey, Jr., Dr. S. Dillon

Ripley II, Dr. Kenneth C. Parkes, and Mr. Aaron M. Bagg. This paper was submitted

to the Zoology Department, Yale University, as a master’s thesis.

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2502 BRONSON ROAD, FAIRFIELD, CONNFXI’ICUT, 16 APRIL 1962